The present invention relates to an ion laser apparatus and a mirror adjusting method therefor.
FIG. 5 shows the overall arrangement of a conventional ion laser apparatus. As shown in FIG. 5, an ion laser apparatus 101 is comprised of a laser oscillator 102, alignment controller 103, and power supply 127. The laser oscillator 102 is comprised of a laser tube 104, a support 105 for supporting the laser tube 104, an output mirror 106, a total-reflecting mirror 107, and a mirror angle adjusting mechanism 108 for adjusting the mirror angle. The alignment controller 103 is comprised of an optical detector 118 for measuring the intensity of a monitor beam 117, an A/D converter 121, an arithmetic controller 122 for controlling stepping motors 120, and motor drivers 123 for driving the stepping motors 120.
The operation of the ion laser apparatus having the above arrangement will be briefly described. A laser beam 115 emitted by the laser tube 104 is reflected by the total-reflecting mirror 107 and transmitted through the output mirror 106. Part of the transmitted laser beam 115 is guided by a beam splitter 116 to the optical detector 118 as the monitor beam 117, while the remaining transmitted laser beam 115 emerges to the outside as the laser beam 115. The optical detector 118 detects the optical intensity of the monitor beam 117. A signal obtained by the optical detector 118 is A/D-converted by the A/D converter 121, is processed by the arithmetic controller 122, and drives the stepping motors 120 through the motor drivers 123 in order to correct the tilt of the total-reflecting mirror 107 or output mirror 106.
The detailed structure and operation of the mirror angle adjusting mechanism 108 will be described.
FIG. 6 shows the mirror angle adjusting mechanism 108 in enlargement.
FIG. 7 shows a stationary plate seen from a side where the stepping motors 120 are disposed. The support 105 where the laser tube 104 is fixed is constituted by an invar rod 109 which is a metal having a low coefficient of thermal expansion, to suppress thermal expansion in the direction of optical path of the laser beam 115. Stationary plates 110 and movable plates 111 are arranged on two ends of the support 105 with tension springs 112 and adjustment screws 114a to 114c, to be parallel to each other. The output mirror 106 and total-reflecting mirror 107 are fixed to the movable plates 111, respectively, through mirror holders 113.
The gap between each stationary plate and the corresponding movable plate is determined by the projecting lengths of the adjustment screws 114a to 114c from the movable plate. The lengths of the projecting portions of the three adjustment screws 114a to 114c and the positions of the three adjustment screws 114a to 114c which are determined by design determine the tilt of the mirror. When the adjustment screw 114b is rotated about, of the adjustment screws 114a to 114c arranged to form a shape L, the adjustment screw 114a located at the pivotal point as the fulcrum, the tilt of the mirror can be changed in the vertical direction. Similarly, when the adjustment screw 114c is rotated, the tilt of the mirror can be changed in the horizontal direction. The tilt of the mirror is adjusted to an arbitrary value by the two adjustment screws 114b and 114c while measuring a laser output, thereby adjusting the laser output to the maximum value. The adjustment screws 114 are driven in the following manner. The arithmetic controller 122 performs arithmetic operation based on an output signal of the monitor beam 117. The obtained operation signal indicating the rotational direction and angle of each motor shaft is sent from the arithmetic controller 122 to the stepping motors 120b and 120c through the motor drivers 123 as the number of pulses necessary for the stepping motors 120b and 120c. As a result, the stepping motors 120b and 120c are rotated, thereby driving the adjustment screws 114.
The procedure of adjusting the mirror angle of the ion laser apparatus having the above arrangement will be described.
FIG. 8 shows a conventional mirror angle adjusting procedure. In the following description, for the sake of descriptive convenience, left and right sides are those obtained when viewed from the reflection side to the exit direction. The stepping motors 120b and 120c, and the adjustment screws 114b and 114c may form reduction gear structures by means of gears. For easy description, a case wherein the reduction ratio is 1:1 will be described. Regarding the tilt angle of the mirror with respect to the rotational angle of the adjustment screw, for the sake of easy description, note that a rotational angle of 1xc2x0 of the adjustment screw corresponds to a change of 0.01xc2x0 of the tilt angle of the mirror.
In step S200, the ion laser apparatus 101 is started by a constant-current operation obtained by controlling a discharge current to a constant value, thereby performing laser oscillation. In step S210, of the laser beam, a monitor beam reflected by the beam splitter 116 is detected. In step S220, the detected data is A/D-converted.
The flow enters the coarse adjustment operation mode (S230), which is the first step of automatic mirror adjustment. First, in step S231, coarse adjustment in the vertical direction is performed. In step S231, the vertical-direction adjustment screw 114b is rotated, and its output change data is acquired. For example, the vertical-direction stepping motor 120b is rotated counterclockwise through a xc2xd turn to rotate the adjustment screw 114b counterclockwise through a xc2xd turn. From this position, the stepping motor 120b is rotated clockwise through a xc2xd turn, while measuring the output data of the laser beam in units of specified angles (the angle is specified by variably changing the pulse count). In this conventional example, an angle of 3xc2x0 is defined as one step (unit). When 60 data corresponding to a xc2xd turn are measured, the stepping motor 120b returns to the initial position. After that, the stepping motor 120b is further rotated clockwise through a xc2xd turn while measuring the output data. Hence, measurement of data on the xc2xd turn from the initial position in each of the clockwise and counterclockwise directions or a total of 1 turn, i.e., 120 output data, is completed. This corresponds to 3.6xc2x0 in mirror angle. FIG. 9 shows a measurement example of the output data.
FIG. 9 shows the characteristics of alignment sensitivity indicating the scan angle width and an output variation width. Generally, the change characteristics of the laser output with respect to a change in mirror angle are called alignment sensitivity characteristics. In FIG. 9, the initial position at the start of a laser is defined as the reference position, and the center of the axis of abscissa is defined as 0. If the width of these characteristics is large, the laser oscillator is not sensitive to a change in mirror angle; inversely, if it is small and forms a sharp shape, the laser oscillator is sensitive to a change in mirror angle. In the case of FIG. 9, the maximum value is located at 90xc2x0 of the counterclockwise rotation of the motor shaft. This is due to the following reason. Since this state is immediately after the laser is started, the temperature in the oscillator has not reached a stable state, so that the maximum value is offset from the optimum angle of the mirror.
The adjustment screws are adjusted on the basis of the measured data. More specifically, the stepping motor 120b is so rotated as to return to the maximum angle of the measured data, and is stopped. In this example, the stepping motor 120b is rotated counterclockwise through 90xc2x0, and then stopped. Coarse adjustment in the vertical direction is thus completed. In this state, a position reached after rotation through 90xc2x0 from the initial position serves as the reference position in the next step. Therefore, data is shifted such that the position of 90xc2x0 in FIG. 9 comes to 0xc2x0 at the center (see FIG. 10). Subsequently, in step S232, coarse adjustment in the horizontal direction is performed in the same manner as in step S231. More specifically, the horizontal-direction stepping motor 120c is rotated, and data is measured. The stepping motor 120c is rotated, on the basis of the measurement data, through an optimum mirror angle with which the maximum output can be obtained. Coarse adjustment in the horizontal direction is thus performed.
The coarse adjustment mode is thus completed. Consecutively, the fine adjustment operation mode in step S240 is performed in order to maintain the optimum mirror angle so as to cope with a temperature rise in the oscillator or a change in the ambient temperature that occurs after the coarse adjustment mode. In both the vertical and horizontal directions, a position in FIG. 10 where the maximum output can be obtained is assumed as the reference 0.
In the same manner as the coarse adjustment mode, the fine adjustment mode is performed by rotating the motor clockwise and counterclockwise from this position to a position where the maximum output within the rotation width can be obtained. If the rotational angle is large, the output varies largely. Thus, the rotational angle in the fine adjustment mode is smaller than that in the coarse adjustment mode. The fine adjustment mode is different from the coarse adjustment mode in this respect.
In the coarse adjustment mode, as shown by the alignment sensitivity characteristics of FIG. 9, the motor is rotated clockwise and counterclockwise through an angle equal to or more than the width of the laser oscillation angle (with a rotational angle of about 230xc2x0 and a mirror angle of 2.3xc2x0). In this example, the motor is rotated clockwise and counterclockwise with a rotational angle of 360xc2x0 and a mirror angle of 3.6xc2x0. In the fine adjustment mode, the motor is rotated in a trial-and-error manner in the vicinity of the peak of the alignment sensitivity characteristics to find the maximum point.
Conventionally, a scan angle width xcex8 is set in advance, and the stepping motor is rotated within this range.
To perform fine adjustment in the vertical direction (S241), the vertical-direction stepping motor 120b and adjustment screw 114b are rotated counterclockwise through xcex8, and are rotated clockwise through xcex8 while measuring the output data of the laser beam in units of specified angles (specified by variably changing the pulse count). In this example, xcex8 data corresponding to xcex8xc2x0 are measured with reference to 1xc2x0 as one step (unit), and the motor 120b is consecutively rotated clockwise through xcex8xc2x0 while measuring the output data. Hence, measurement of 2xcex8 pieces of output data corresponding to xcex8xc2x0 in each of the clockwise and counterclockwise directions, i.e., a total of 2xcex8xc2x0, is completed.
Subsequently, the motor is rotated from the initial position through an angle corresponding to the maximum value of the measurement data of xcex8xc2x0 in both the clockwise and counterclockwise directions, and is stopped, to adjust the adjustment screw. Thus, one cycle of vertical-direction fine adjustment is ended. When the adjustment screw is moved in this manner, the laser output varies accordingly. The magnitude of variations changes within an output variation width xcex41, as indicated by the output characteristics with respect to the mirror angle shown in FIG. 11. Then, horizontal-direction fine adjustment in step S242 is performed by moving the horizontal-direction stepping motor 120c in the same manner as in step S241. The horizontal-direction motor 120c is rotated through an angle corresponding to the maximum value of the measurement data, to adjust the corresponding adjustment screw. Thus, one cycle of horizontal-direction fine adjustment is ended.
If the respective elements of the laser oscillator 102 do not vary, further adjustment is not needed. In practice, however, since an alignment error occurs accompanying a temperature change, the fine adjustment operation in step S240 must be repeated until the laser apparatus 101 is stopped. With the above operation, an error in mirror angle caused by the temperature change is constantly corrected in order to set an optimum alignment state necessary for obtaining the maximum output at that point. The maximum output value accompanies an output variation width xcex41.
The automatic mirror adjusting mechanism in the conventional ion laser apparatus has the following problems. The ion laser apparatus 101 is a laser unit that excites ions by performing discharge of several 10 amperes (to be described as A hereinafter) in a small hole called a thin pipe 124, in the laser tube 104, which has an inner diameter of several millimeters (to be described as mm hereinafter) and a length of several 100 mm, thereby producing laser oscillation. A discharge plasma of several 10 A sputters the inner surface of the thin pipe 124 to denature its material, accordingly changing the shape of the inner surface. As a material that can endure sputtering, beryllium oxide having excellent sputtering resistance is generally employed. Under the discharge current condition of as large as 50 A, however, the thin pipe 124 made of beryllium oxide is naturally denatured and deforms by sputtering. Also, a Brewster window 125 of the laser tube 104 is degraded by the ultraviolet rays emitted by the plasma discharge, and its transmission characteristics are accordingly decreased.
It is known that degradation of the component material over time as described above changes the alignment sensitivity to have a sharp peak. When the alignment sensitivity becomes sharp due to this change in the laser tube 104, the conventional automatic mirror adjusting mechanism performs mirror angle adjustment while maintaining the constant scan angle width of xcex8xc2x0 which is set initially. Since the scan angle width of xcex8xc2x0 is excessively large, the output variation width increases from xcex41 to xcex42, as shown in FIG. 12.
Various types of automatic mirror adjusting mechanisms have been proposed to solve these problems. For example, according to Japanese Patent Laid-Open No. 5-37050, to accurately obtain the output value and to perform correct determination, the number of times of the sampling operation is increased. A plurality of data corresponding in number to the sampling operation times are averaged to determine the magnitude of the output value. In this invention, however, the number of times of the sampling operation is increased to merely improve the reliability of the data, and the alignment sensitivity itself stays sharp. Therefore, the problem of sharp alignment sensitivity described above cannot be solved.
Japanese Patent Laid-Open No. 9-153654 discloses an invention that enables adjustment with a very small angle by using an electrostrictive element. According to this technique, the very small displacement amount of the angle of the laser mirror is adjusted by adjusting the pulse width of the voltage pulse in advance. A very small displacement amount of the angle of the mirror is synonymous with the preset scan angle width xcex8. The adjustment range does not change in accordance with the alignment sensitivity that changes over time. Therefore, the problem described above cannot be solved.
According to the technique disclosed in Japanese Patent Laid-Open No. 5-21885, in the operation of light feedback mode or light mode generally referred to in the ion laser apparatus, a discharge current is used when performing detection and control. Referring to the graph of the operation characteristics shown in this reference, when the transverse mode is a single mode, the flat portion of the peak forms a sharp hill, while the discharge current forms a deep bottom. As a result, with the control operation of this technique, a small change in mirror angle causes a sharp increase/decrease in the discharge current, and discharge is sometimes discontinued, which is a problem.
The present invention has been made in view of the above problems, and has as its main object to provide an ion laser apparatus and a mirror angle adjusting method therefor, with which the output variation width of a laser beam can be set within a predetermined range even when the characteristics of a constituent component such as a laser tube change over time.
In order to achieve the above object, according to the present invention, there is provided an ion laser apparatus comprising a laser tube, first and second mirrors disposed to sandwich the laser tube, a mirror angle adjusting mechanism for adjusting an angle of at least one of the mirrors while scanning the mirror within a predetermined angle width, and an alignment controller for determining a scan angle width of the mirror in accordance with a light intensity distribution of a laser beam such that a variation value of the laser beam output from the laser tube falls within a predetermined width.